Field of the Invention
The present invention relates to an interferometer, a lithography apparatus, and a method of manufacturing an article.
Description of the Related Art
A surface shape measurement apparatus using a white light interferometer divides white light emitted by a light source into two by a beam splitter, and makes the two beams obliquely incident on an object to be detected and a reference surface. The measurement apparatus superimposes, by a beam splitter, the measurement light and reference light respectively reflected by the object and reference surface. After that, the measurement apparatus detects a peak position for the interfering light acquired by an image sensor, and measures the surface position of the object. By two effects of a short coherence length unique to a signal of white light interference, and a high reflectance of the substrate surface by oblique incidence, the surface shape measurement apparatus using the white light interferometer can suppress a measurement error arising from film thickness unevenness on the substrate or interference between the front and back surfaces caused by a pattern. US Patent Application Publication No. 2007/0086013 discloses a surface shape measurement apparatus using a spectral white light interferometer.
The measurement apparatus disclosed in US Patent Application Publication No. 2007/0086013 acquires, by using a spectrometer and two-dimensional image sensor, a signal obtained by interference between measurement light traveling from an object to be detected and reference light traveling from a reference surface. A spectral signal obtained from the two-dimensional image sensor via the spectrometer is Fourier-transformed by an arithmetic means. The peak position of the acquired white light interference signal in the time domain is detected to measure the surface position of the substrate. Therefore, the measurement apparatus disclosed in US Patent Application Publication No. 2007/0086013 can quickly measure a surface shape without mechanically scanning an object to be detected in the Z direction at each measurement point on the X-Y plane.
In general, however, a surface shape measurement apparatus using a white light interferometer with a short coherence length has problems of low spatial coherence and a narrow measurement range. The spatial coherence means coherence on a cross section along the optical axis. The coherence includes temporal coherence in addition to the spatial coherence. The spatial coherence represents coherence on the cross section along the optical axis, whereas the temporal coherence represents coherence in the optical axis direction, that is, the range of an interfering optical path length. A problem to be solved by the present invention is directed to the spatial coherence. In an oblique-incidence interferometer, if the position of an object to be detected shifts in the direction of height, measurement light shifts with respect to reference light on the cross section along the optical axis in principle, generating a positional shift (to be referred to as a vertical shift hereinafter). When a low-coherent light source such as a halogen light source or white LED is used, the spatial coherence decreases owing to the vertical shift of measurement light with respect to reference light. Along with the decrease in contrast, the measurement accuracy drops.
To suppress the decrease in spatial coherence along with the vertical shift of measurement light with respect to reference light, there is a method of arranging an aperture stop with a low numerical aperture at the pupil position. The aperture stop with the low numerical aperture is arranged to increase the blur amount of a point image and increase the area by which reference light and measurement light overlap each other on the image plane. This can suppress the decrease in spatial coherence along with the vertical shift of measurement light with respect to reference light. However, when the aperture stop with the low numerical aperture is arranged, the signal strength decreases, and the measurement accuracy drops along with a decrease in the S/N ratio of the signal. Thus, the oblique-incidence white light interferometer has a problem that the measurement accuracy drops owing to the decrease in signal strength when the decrease in spatial coherence along with the vertical shift of measurement light with respect to reference light is suppressed.
US Patent Application Publication No. 2007/0086013 discloses an arrangement in which an aperture stop having a rectangular aperture is arranged at the pupil position in a spectral white light interferometer. However, this literature does not describe the relationship between the shorter side direction of the rectangular aperture and the direction of the line of intersection of the incident surface and beam cross section, and the relationship between the direction of wavelength resolution, and the direction of the line of intersection of the incident surface and beam cross section in the spectrometer. These relationships are very important to implement high-accuracy measurement in the spectral white light interferometer. Hence, the contents of the description in US Patent Application Publication No. 2007/0086013 are insufficient to measure the surface shape of an object to be detected at high accuracy.